This invention relates to television horizontal deflection circuits which regulate the deflection magnitude against changes in the supply voltage.
It is advantageous both from the point of view of cost and because of weight consideration to derive the direct operating voltages for a television receiver by rectification of the alternating current power from the ac power lines without the use of a power-line frequency transformer. The voltages so derived, however, may not be at a value suited for use in low-voltage stages of a receiver without excessive waste of power. Also, the ac input voltage varies significantly from time to time and is not suited for powering stages sensitive to such changes. In particular, the horizontal output stage, if supplied with unregulated voltage, may scan the kinescope to form a raster of varying width. It is desirable to regulate the horizontal scanning width by control of the scanning energy, and to step down the voltage derived from rectification of the ac power for use in low-voltage parts of the receiver.
A horizontal deflection circuit which regulates the deflection amplitude against changes in the unregulated direct supply voltage is described in an article entitled "A New Horizontal Deflection Circuit" by Peter L. Wessel, published in the IEEE Transactions on Broadcast and Television Receivers, Volumn BTR-18, August 1972. This horizontal deflection circuit is described as a combined switched-mode power supply and trace-driven deflection circuit, and uses a single switch transistor. Unregulated direct voltage is applied to the collector of the switching transistor through the primary of a transformer, the secondary of which is coupled to the deflection winding through a diode poled to conduct and thereby supply energy to the retrace capacitor during the retrace interval. The switch transistor is supplied with base drive from a control circuit which provides for switch transistor conduction during the last half of the horizontal trace interval, and which also provides for regulation of the deflection energy by controlling the time during the first half of trace during which the switch transistor is conductive.
The described Wessel circuit requires a complex control circuit which is used solely for control of the deflection energy. Also, the transformer is operated with an average direct current flow in both the primary and secondary windings, which causes the magnetic core to be operated away from the point of maximum permeability and requires the use of a larger core than would be the case without an average direct current flow.
U.S. Pat. No. 3,967,182 issued on June 29, 1976 in the name of Howard M. Scott describes a regulated switched mode power supply with multiple outputs in which the unregulated direct voltage input is chopped by a pair of switching transistors and applied to the primary of a transformer. The output voltages are taken from secondary windings of the transformer by half-wave rectification and filtration. The output voltages are maintained constant against changes in the unregulated direct voltage input by a control circuit coupled to the switching transistors which varies the duty cycle or ON-OFF periods of the switching transistors so as to maintain the average primary winding current at zero and also to maintain the secondary voltage constant during the conduction interval of the half-wave rectifiers.
The aforementioned Scott arrangement provides for a reduction in transformer size by virtue of the low primary winding direct current, and also provides multiple regulated output voltages which, by transformer action, may be greater or less than the input voltage. However, where the horizontal output stage is one of the loads being energized, the secondary winding supplying the half-wave rectified power thereto has a substantial average direct-current component which tends to increase the required transformer size. Also, such a combined arrangement is relatively complex.
It is desirable to have a simple structure providing regulated energy for horizontal deflection and regulated voltages for high or low-voltage portions of the receiver. It is also desirable to have reduced transformer size resulting from reduction in average direct current in the transformer windings.